Abstract
Interocular summation has been measured using a wide variety of stimuli, measurements, and models. Our goal was to develop an intuitive and robust measure of interocular summation that reflects naturalistic (non-rivalrous) conditions and provides a direct measure of the perceptual experience of the observer. Observers fixated a Gabor (2 cpd; 4 deg radius, orientation rotating at 1 deg/sec to minimize adaptation effects) through a stereoscope. Gabor contrast was slowly modulated at 1/8 Hz in one eye, and 1/6 Hz in the other. Subjects dynamically reported perceived contrast over time by manipulating a Thrustmaster Pro joystick.
With less than an hour of data per subject it was possible to estimate individual contrast response functions for each eye. Data were well fit by a simple model that indicated binocular summation varied across individuals, ranging from approximately quadratic summation to a max rule. The ratio of the best fitting gain parameters for the left and right eye were closely correlated with interocular contrast ratios measured using a previously-established interocular phase-offset technique (Kwon et al. 2014), including in participants with binocular dysfunction. Our dynamic perceived contrast task provides an intuitive, rapid and robust method for characterizing binocular summation in typical and atypical binocular vision such as amblyopia and strabismus, and is suitable for measuring BOLD responses to each eye independently in an fMRI scanner.